Segmented thermoplastic polyester copolymers are known as a elastic material with improved mechanical properties and heat resistance. Particularly, their heat resistance is characteristic. Generally, no thermoplastic elastomer other than segmented thermoplastic polyester copolymers can be employed at elavated temperatures as high as 120.degree. C or greater.
For example, reference is made to the bottling of foodstuffs. After the bottling and sealing, each foodstuff in a bottle is usually sterilized at elevated temperatures. Thermal properties of a bottle crown liner are critical during the sterilizing treatment. If a material for the crown liner is poor in heat resistance, the crown liner is so deformed by the application of heat as to break the air-tightness required between the bottle top and the crown liner. While segmented thermoplastic polyester copolymers seem to be a useful lining material due to their excellent heat resistance, it has been found that such segmented polyester copolymers still have some drawbacks. The crown cap is provided with a liner on the inner side thereof by extruding a molten resin as a strand in shape, cutting the molten strand into molten particles, applying molten particles on the inner side of the crown cap, and press molding the molten particles therein. The strand should be cut before it solidifies. The inventors have found that a strand made of molten, segmented thermoplastic polyester copolymer can not be easily cut, that is to say, a segmented thermoplastic polyester copolymer cannot be easily cut while hot. The strand made of the segmented polyester copolymer can not be completely cut into particles because of stringing between each particle. This may bring about reduction in the operating efficiency of the copolymer during molding. Furthermore, we have found that the segmented thermoplastic polyester copolymer is not adhesive enough, particularly to metal. In practice, most segmented polyester copolymers may not be applied to metal or any other material as a coating because such a coating is easily stripped or peeled.
On the other hand, it is known that polyolefins exhibit poor heat resistance and adhesion, as well as low environmental stress cracking resistance. The latter results from the fact that there remains a comparatively larger residual strain in a molded piece of polyolefin, which gives the differences in coefficient of thermal expansion between different locations when exposed to a certain liquid or vapor thereof, causing cracks in the molded piece.